There have been various disclosures and teachings to the use of cation-exchange membranes in electrolytic processes, e.g., chlor-alkali cells where aqueous NaCl is electrolyzed by an electric current between anode and cathode and where the electrolyte is separated into anolyte and catholyte portions by a substantially hydraulically-impermeable membrane. In chlor-alkali membrane cells caustic (NaOH) collects in the catholyte (substantially salt-free), hydrogen gas is evolved at the cathode, and chlorine gas is evolved from the NaCl-rich anolyte at the anode. Ordinarily the hydrogen and chlorine products are evolved and removed from the cells at about atmospheric pressure, though there are some cells which have been disclosed as pressure cells wherein the chlorine is removed under pressure as a liquid, or where the hydrogen and chlorine are removed as gases at above atmospheric pressure.
The cation-exchange membranes of particular interest in the present invention are those prepared from fluoropolymers and which contain pendant sulfonic acid functional groups and/or carboxylic acid functional groups. The sulfonic acid groups and carboxylic acid groups may derive from polymers prepared from monomers which contain such groups or may be substituted onto the fluoropolymer after polymerization. Hydrolyzed copolymers of a prefluorinated hydrocarbon (e.g., tetrafluoroethylene) and fluorosulfonated perfluorovinyl ethers (e.g., FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF.dbd.CF.sub.2 and FSO.sub.2 CF.sub.2 CF.sub.2 OCF.dbd.CF.sub.2) having eq. wts. in the range of about 800 to 2000 are of particular interest. Another fluoropolymer of particular interest is sulfostyrenated perfluoroethylene-propylene, which is prepared by styrenating an FEP (fluorinated ethylene-propylene) copolymer, then sulfonating the polymer. Also of interest are fluoropolymers which contain pendant R--COOH groups having eq. wts. in the range of about 500 to 1500; these polymeric membranes may, alternately, also contain sulfonic acid, R--SO.sub.3 H groups. Layered membranes having functional groups in one layer which are predominantly, or substantially all, of the carboxylic type, and in another layer which are predominantly, or substantially all, of the sulfonic acid type, are of interest. Methods of making such polymers are known to practitioners of these arts and the making of them is not within the purview of the present invention. It is within the purview of the present invention to treat such polymer membranes with certain amines, under certain conditions, to form easily disassociable amine salts whereby the amines are leached out during use in an aqueous electrolyte, thereby returning the sulfonic and carboxylic functional groups to their original acid state.
The easily disassociable salt groups employed in the present invention are not the same as groups formed by reacting a base, such as an amine, with acid halide groups (e.g., sulfonyl halide) to form amides, such as sulfonamides or carboxamides. Such amide groups are much more difficult to be returned to their original acid forms by an aqueous leaching.
It is known in the arts that fluoropolymers containing pendant sulfonyl halide groups may be saponified to convert the sulfonyl halide groups to sulfonic acid functionality groups. This may be demonstrated by the following equation, where R represents the portion of the polymer molecule to which the group is attached: ##STR1##
The R--SO.sub.3 Na, in acidic medium, becomes R--SO.sub.3 H.
It is also known that fluoropolymers containing R--COOH groups can be converted to carboxylate groups, e.g., R--COONa, or carboxamides, e.g., R--CONH.sub.2, with R representing attachment to the fluoropolymer.
In contradistinction thereto, the present invention relies on preparing easily disassociable amine salts which may be represented by the formulae:
R--SO.sub.3 H.(amine) and R--COOH.(amine), where (amine) represents a primary, secondary, or tertiary amine as hereinafter described. The amine is not reacted to the point of giving off a mole of H.sub.2 O to form a sulfonamide or carboxamide.
Dimensional stability of the cation-exchange membranes is a recognized problem, sometimes with regard to swelling (which can cause sagging) or deswelling (which can cause tautness leading to tearing, cracking, or splitting). The use of various fluoropolymers as cation-exchange membrane material in electrolytic cells, and the problems associated with dimensional instability are a matter of record as shown, e.g., by U.S. Pat. Nos. 3,985,631 and 4,000,057. These patents are believed to be representative of the most relevant prior art.
U.S. Pat. No. 3,985,631 proposes a remedy for the problems associated with dimensional instability of cation-exchange membranes; the remedy includes pretreatment with aqueous NaOH to introduce hydrated alkali metal ions throughout the membrane.
U.S. Pat. No. 4,000,057 also proposes a remedy for the problems associated with dimensional instability of cation-exchange membranes; the remedy includes preswelling of the membrane with an aqueous solution containing one or more solutes of the group consisting of NaCl, ethylene glycol, glycerine, sodium hydroxide, synthetic organic detergents, lower alkanols, higher fatty acids, organic acids, mineral acids, sequestrants, organic solvent materials, sorbitol, mannitol, polyhydric alcohols, and pentaerythritol.